1. Field of the Invention
This invention relates to a method and a device configuration for determining the corneal thickness of an eye. In particular, this invention relates to a method and a device configuration for determining the corneal thickness of a human eye by means of light rays, first light rays being projected onto and into the cornea, and second light rays, reflected by the cornea, being registered for determining the corneal thickness.
2. Description of the Related Art
Besides the use of ultrasound for determining a human eye""s corneal thickness averaged over a large range, pachymetry attachments for slit lamps are being used today to determine the corneal thickness by means of light rays. By means of a pivotable, plane-parallel plate in the path of the examination beam of a slit lamp microscope, such an attachment generates two offset half images of the light section with the cornea. By pivoting the plane-parallel plate, the offset light sections can be made congruent, and a measured value corresponding to the pivoting can be read for the cornea. Such a measurement takes place only at one point of the cornea, requires manual skill, and is difficult to reproduce since the position of the measurement point is not defined. The drawback of these methods for determining the corneal thickness is, in particular, that they are not suitable for determining corneal thickness with a high area resolution, i.e. they do not make possible determination of the corneal thickness area-wise, or in a way that completely covers the area. Consequently local deviations of the corneal thickness cannot be registered by means of these methods, which is extremely risky, and therefore disadvantageous, in particular for surgical procedures (e.g. refractive surgery).
Described in U.S. Pat. No. 5,512,965 is a device and a method based on a modified slit lamp, and by means of which a three-dimensional print-out of the cornea surface and of the respective local corneal thickness can be generated. The device according to U.S. Pat. No. 5,512,965 comprises a modified slit lamp, the projection slit of which is curved for improvement of the depth of focus, a television camera with associated lens systems, electronic circuitry for selecting and quantifying television pictures taken, as well as a control mechanism for moving the light section generated by the slit lamp. According to U.S. Pat. No. 5,512,965, a multiplicity of digitally coded pictures of the recorded light section are evaluated by means of software, measurements of individual light sections being put together via reference marks of the eye (limbus and Purkinje images of the exit pupil and highlights of the slit projector) for overall measurement. According to the method described in U.S. Pat. No. 5,512,965, the patient with the eye to be measured must focus his gaze on a target object so that eye movements that are too great can be avoided during the measurement. According to U.S. Pat. No. 5,512,965, the focus of the television camera during the measurement is updated to the cornea with an additional expense. Since the refraction of the beams, incident in the cornea, of the light section is known to depend upon the local surface inclination of the cornea, the inclination of the cornea must be measured with an additional measuring effort in order to determine the corneal thickness, in the method according to U.S. Pat. No. 5,512,965. The method according to U.S. Pat. No. 5,512,965 requires moreover a complex calibration with each measurement since the angle of illumination, the viewing angle as well as the object spacing continually change. The method according to U.S. Pat. No. 5,512,965 requires above and beyond this a complex alignment of the patient with additional marks. Owing to the high expense in technical devices, the method according to U.S. Pat. No. 5,512,965 is implemented only in the form of a standing apparatus, which has the drawback that the method is not usable for patients lying down, for example in the operating room.
The significance in ophthalmology of an area-wise corneal measurement with high resolution is clear, especially for surgical procedures on the cornea. In particular where the cornea is cut at a predefined depth or where the cornea is penetrated to a predefined depth, precise knowledge of the local corneal thickness is important. Examples of such surgical procedures are radial keratotomy, LASIK (laser in situ keratomileusis) and ALK (automated lamellar keratoplasty). Whereas in the radial keratotomy, thin superficial slits are cut into the cornea, in LASIK and ALK thin layers of the cornea are cut in flattened state of the cornea. From a medical point of view, therefore, it is extremely important to know the relevant corneal thickness prior to the operative procedure. Only with knowledge of the corneal thickness can it be ensured that an incision is not too deep and that no complications arise.
It is an object of this invention to propose a method and a device configuration for determining the corneal thickness of an eye which at least do not have certain of the above-described drawbacks of the state of the art, and which make possible, in particular, a determination of the relevant corneal thickness of a human eye area-wise with a high area resolution. It is a further object of the present invention to reduce the expense in technical devices for determination of the corneal thickness of an eye, in order to make possible, in particular, the achievement of hand-held devices for area-wise corneal thickness measurement with a high area resolution.
These objects are achieved, according to the present invention, through the elements of the independent claims. Further preferred embodiments follow moreover from the dependent claims and from the specification.
In particular, these objects are achieved by means of the invention in that a contact element is placed on at least one contact area of the cornea, and light waves are projected through the contact element and into the cornea by a light source. Light waves, which are reflected by the cornea and the contact element, can then be registered by registering means, and a simple measurement of the thickness of the cornea is made accessible on the basis of the defined measurement conditions created by the contact element. Determination of the surface inclination of the cornea, or the curved design of projection slits to meet the Scheimpflug condition, or an autofocus system for the recording means, for example, become unnecessary. Moreover the contact element makes it possible to prevent more easily undesired eye movements and deformations of the eye, for example through eye focusing, during the measurement.
In a preferred embodiment variant, the light waves, which are emitted by the light source at least in points in a two-dimensional measuring area of the contact area of the cornea on which the contact element is placed, are projected onto and into the cornea, and the corneal thickness is determined at one or more places in the measuring area. The corneal thickness can thereby be determined with a high area resolution, for example with complete coverage within the measuring area.
In a preferred embodiment variant, a contact element is used which has a predefined thickness, and the thickness of the contact element is co-used for determination of the corneal thickness. With known local thickness of the contact element using optical methods, which are based on specular reflection at the boundary surfaces of the contact element and of the cornea, and in which the thickness of the contact element can be co-established on the basis of light waves reflected by the contact element, no knowledge is required of the incidence angle of light rays at the boundary surfaces, and only the refraction indexes of the contact element and of the cornea must be known. This embodiment variant thus has the advantage that the time, effort and expense for determining the local surface inclination of the cornea in the known methods is eliminated. If, moreover, the contact element has the same refraction index as the cornea, only the ratio of the lengths of the paths has to be measured which the light waves projected into the contact element and into the cornea have taken in the contact element, or respectively in the cornea, and also methods based on light scattering or fluorescence in the cornea and in the contact element or on diffuse reflection at their boundary surfaces do not require then any knowledge of the location surface inclination. Furthermore the imaging scales, i.e. the relationship of object size to picture size, do not need to be known in order to determine the corneal thickness. Thus, for example, corneal thickness measurements can be carried out in a conventional slit lamp image with the aid of a contact lens of known thickness and with the same refraction index as the cornea.
In an embodiment variant, the cornea is brought into a defined state by means of the contact surface of the contact element, i.e. the shape and/or position of the cornea is determined by means of the contact surface of the contact element. On the basis of the defined state, which is known a priori or is determinable, depending upon the embodiment, the local surface inclination of the cornea and of the local imaging scale can be determined without high expense in technical devices. With optical methods based on specular reflection at boundary layers, the lengths of the courses of the light waves, which are dependent upon the surface inclination, in the cornea can thus be determined, and from this, the corneal thickness (perpendicular to the surface of the cornea) can be determined with knowledge of the refraction index of the cornea and of the contact element. With methods with light scattering or fluorescence in the cornea or diffuse reflection at the boundary surfaces, required in addition to the angle of incidence for the incoming light rays, are the exit angle (refraction angle) of the light rays out of the contact element as well as knowledge of the local contact element thickness and of the refraction index of the contact element and of the cornea. Moreover, in the case of non-telecentric imaging by the registering means, the contact element thickness and the imaging scale must be known. If the contact element is suitably designed, a multiplicity of these parameters can be determined simply and efficiently.
In a preferred embodiment variant, a plane-parallel contact element is used, the thickness of which is co-determined during the measurement. The use of a plane-parallel contact element has proven to be especially advantageous also for methods with light scattering or fluorescence or diffuse reflection since the above-mentioned required measurement values, respectively measurement parameters, can be obtained with a significantly lower expense in technical devices than in known methods. The use of a plane-parallel contact element for placement on the cornea makes possible embodiment variants in which the angle of illumination (angle of incidence) and the viewing angle and/or the object spacing can be kept constant (with shift, parallel to the contact element, of the measuring apparatus containing the light source and the registering means), so that a costly calibration of the measuring device configuration is eliminated. Moreover the patient does not need to be aligned in a complex way to the measuring apparatus.
In an embodiment variant, the contact element is fixed to the cornea or to the eye, for example by means of partial vacuum (suction ring). This has the advantage that a firmly defined state of the cornea is achieved and that the measuring process can be carried out more slowly since no relative movements between the eye and the contact element occur during the measurement. In addition, the fixing of the contact element to the cornea or to the eye prevents eye pressure fluctuations, for example caused by the pulse, and focusing of the eye from changing the shape and the position of the cornea.
In an embodiment variant, a semi-transparent contact element is used. A semi-transparent contact element causes incident light to be scattered so that optical methods based on light scattering or fluorescence or diffuse reflection can be applied.
In an embodiment variant, a contact element is used the refraction index of which corresponds to the refraction index of the cornea. This embodiment variant has the advantage that the corneal thickness can be determined independently of the incidence angle, or respectively the reflecting angle, of the light rays as well as independently of the refraction index for the contact elements and the cornea.
In an embodiment variant, the light source and/or the registering means are disposed according to the Scheimpflug condition in order to prevent defocusing. Through the Scheimpflug configuration sharper images can be generated, with object plane and picture plane inclined toward one another, than is possible with other configurations, for example a perpendicular disposition of the picture plane to the optical axis of the imaging system.
In an embodiment variant, the light waves projected by the light source are beamed on the cornea with a constant angle of incidence. Determination of the corneal thickness can thus be simplified since the angle of incidence has to be determined only once.
In an embodiment variant, the reflected light waves are registered at a constant viewing angle. Determination of the corneal thickness can thus be simplified since the viewing angle has to be determined only once.
In an embodiment variant, the light source is moved at a constant distance from the contact area of the contact element. The advantage of this embodiment variant is that a higher resolution, or respectively greater precision of measurement can be achieved since there are no vertical relative movements between the cornea and the light source, and light waves can be very finely focused.
In an embodiment variant, the registering means are moved at a constant distance from the contact area of the contact element. The advantage of this embodiment variant is that a higher resolution, or respectively greater precision of measurement can be achieved since there are no vertical relative movements between the cornea and the registering means, and the depth of focus area of the registering means can be designed small and thus the optical resolution high.
In an embodiment variant, the optical measuring apparatus, which comprises the light source and the registering means, is connected to the contact element, the light source and/or the registering means being movable relative to the contact element. If this connection is fixed only during the position determination of the contact element and the subsequent measurement, detachable contact elements can be used, which have different predefined thicknesses, for example, and/or are disposable.